Laser propulsion

Laser propulsion is a form of beam-powered propulsion where the energy source is a remote (usually ground-based) laser system and separate from the reaction mass. This form of propulsion differs from a conventional chemical rocket where both energy and reaction mass come from the solid or liquid propellants carried on board the vehicle.

The basic concepts underlying a photon-propelled "sail" propulsion system were developed by Eugene Sanger and the Hungarian physicist George Marx. Propulsion concepts using laser-energized rockets were developed by Arthur Kantrowitz and Wolfgang Moekel in the 1970s.[1] An exposition of Kantrowitz's laser propulsion ideas were published in 1988.[2]

Laser propulsion systems may transfer momentum to a spacecraft in two different ways. The first way uses photonradiation pressure to drive momentum transfer and is the principle behind solar sails and laser sails. The second method uses the laser to help expel mass from the spacecraft as in a conventional rocket. This is the more frequently proposed method, but is fundamentally limited in final spacecraft velocities by the rocket equation.

A laser-pushed lightsail is a thin reflective sail similar to a solar sail, in which the sail is by being pushed by a laser, rather than the sun. The advantage of lightsail propulsion is that the vehicle does not carry either the energy source or the reaction mass for propulsion, and hence the limitations of the Tsiolkovsky rocket equation to achieving high velocities are avoided. Use of a laser-pushed lightsail was proposed initially by Marx in 1966,[3] as a method of Interstellar travel that would avoid extremely high mass ratios by not carrying fuel, and analyzed in detail by physicist Robert L. Forward in 1989.[4] Further analysis of the concept was done by Landis,[5][6] Mallove and Matloff,[7]Andrews[8] and others.

The beam has to have a large diameter so that only a small portion of the beam misses the sail due to diffraction and the laser or microwave antenna has to have a good pointing stability so that the craft can tilt its sails fast enough to follow the center of the beam. This gets more important when going from interplanetary travel to interstellar travel, and when going from a fly-by mission, to a landing mission, to a return mission. The laser may alternatively be a large phased array of small devices, which get their energy directly from solar radiation.

The laser-pushed sail is proposed as a method of propelling a small interstellar probe by the Breakthrough Starshot project.

Metzgar and Landis proposed a variant on the laser-pushed sail, in which the photons reflected from the sail are re-used by re-reflecting them back to the sail by a stationary mirror; a "multi-bounce laser-based sail."[9] This amplifies the force produced by recycling the protons, resulting in considerably higher force produced from the same laser power

A photonic laser thruster (PLT) is a further adaptation of the multi-bounce laser sail, in which an active laser medium is put in the resonant optical cavity formed between the two mirrors.[10] PLT is predicted to be able to provide the thrust to power ratio (a measure of how efficient a thruster is in terms of converting power to thrust) approaching that of conventional thrusters, such as laser ablation thrusters and electrical thrusters.

There are several forms of laser propulsion in which the laser is used as an energy source to provide momentum to propellant that is carried on board the rocket. The use of a laser as the energy source means that the energy provided to the propellant is not limited by the chemical energy of the propellant.

The laser thermal rocket is a thermal rocket in which the propellant is heated by energy provided by an external laser beam.[11][12] The beam heats a solid heat exchanger, which in turn heats an inert liquid propellant, converting it to hot gas which is exhausted through a conventional nozzle. This is similar in principle to nuclear thermal and solar thermal propulsion. Using a large flat heat exchanger allows the laser beam to shine directly on the heat exchanger without focusing optics on the vehicle. The HX thruster has the advantage of working equally well with any laser wavelength and both CW and pulsed lasers, and of having an efficiency approaching 100%. The HX thruster is limited by the heat exchanger material and by radiative losses to relatively low gas temperatures, typically 1000 - 2000 C. For a given temperature, the specific impulse is maximized with the minimum molecular weight reaction mass, and with hydrogen propellant, that provides sufficient specific impulse as high as 600 – 800 seconds, high enough in principle to allow single stage vehicles to reach low Earth orbit. The HX laser thruster concept was developed by Jordin Kare in 1991;[13] a similar microwave thermal propulsion concept was developed independently by Kevin L. Parkin at Caltech in 2001.

A variation on this concept was proposed by Prof. John Sinko and Dr. Clifford Schlecht as a redundant safety concept for assets on orbit.[14] Packets of enclosed propellants are attached to the outside of a space suit, and exhaust channels run from each packet to the far side of the astronaut or tool. A laser beam from a space station or shuttle vaporizes the propellant inside the packs. Exhaust is directed behind the astronaut or tool, pulling the target towards the laser source. To brake the approach, a second wavelength is used to ablate the exterior of the propellant packets on the near side.

Material is directly removed from a solid or liquid surface at high velocities by laser ablation by a pulsed laser. Depending on the laser flux and pulse duration, the material can be simply heated and evaporated, or converted to plasma. Ablative propulsion will work in air or vacuum. Specific impulse values from 200 seconds to several thousand seconds are possible by choosing the propellant and laser pulse characteristics. Variations of ablative propulsion include double-pulse propulsion in which one laser pulse ablates material and a second laser pulse further heats the ablated gas, laser micropropulsion in which a small laser on board a spacecraft ablates very small amounts of propellant for altitude control or maneuvering, and space debris removal, in which the laser ablates material from debris particles in low Earth orbit, changing their orbits and causing them to reenter.

A high energy pulse focused in a gas or on a solid surface surrounded by gas produces breakdown of the gas (usually air). This causes an expanding shock wave which absorbs laser energy at the shock front (a laser sustained detonation wave or LSD wave); expansion of the hot plasma behind the shock front during and after the pulse transmits momentum to the craft. Pulsed plasma propulsion using air as the working fluid is the simplest form of air-breathing laser propulsion. The record-breaking Lightcraft, developed by Leik Myrabo of RPI (Rensselaer Polytechnic Institute) and Frank Mead, works on this principle.

Another concept of pulsed plasma propulsion is being investigated by Prof. Hideyuki Horisawa.[18]

A continuous laser beam focused in a flowing stream of gas creates a stable laser sustained plasma which heats the gas; the hot gas is then expanded through a conventional nozzle to produce thrust. Because the plasma does not touch the walls of the engine, very high gas temperatures are possible, as in gas core nuclear thermal propulsion. However, to achieve high specific impulse, the propellant must have low molecular weight; hydrogen is usually assumed for actual use, at specific impulses around 1000 seconds. CW plasma propulsion has the disadvantage that the laser beam must be precisely focused into the absorption chamber, either through a window or by using a specially-shaped nozzle. CW plasma thruster experiments were performed in the 1970s and 1980s, primarily by Dr. Dennis Keefer of UTSI and Prof. Herman Krier of the University of Illinois at Urbana-Champaign.

For spacecraft, laser electric propulsion is considered as a competitor to solar electric or nuclear electric propulsion for low-thrust propulsion in space. However, Leik Myrabo has proposed high-thrust laser electric propulsion, using magnetohydrodynamics to convert laser energy to electricity and to electrically accelerate air around a vehicle for thrust.